Regulations of exhaust gas of vehicles have been strengthened with recently increased environmental consciousness, so that various investigations have been conducted to improve engine systems and to simultaneously improve performances of exhaust-gas purifying catalysts for purifying exhaust gas. Exhaust-gas purifying catalysts include substrates (such as alumina (Al2O3)) having surfaces carrying thereon noble metal particles (such as platinum (Pt) or palladium (Pd)), and oxidize harmful components (such as unburnt hydrocarbons (HC) and carbon monoxide (CO)) contained in exhaust gas by noble metal particles, thereby converting harmful components into harmless components (water and gases). Since noble metal elements as catalytic active components are extremely expensive elements having a fear of resource exhaustion, it is getting difficult to use a large amount of noble metal. There has been thus conducted such an endeavor to bring noble metal into fine particles thereby increasing surface areas and thus increasing contacting areas between exhaust gas and noble metal, to thereby decrease a usage amount of noble metal.
Examples of techniques for bringing noble metal into fine particles include a reversed micelle (microemulsion) method. The reversed micelle method is to firstly mix: a surfactant; and an aqueous solution containing a catalytic active component (such as noble metal element); into an organic solvent. Subsequently, there is prepared reverse micellar solution including the organic solvent each containing the aqueous solution including the noble metal, followed by precipitation of the noble metal and reduction or insolubilization of the precipitated noble metal, thereby depositing the noble metal brought into fine particles within reverse micelles.
Japanese Patent Application Laid-Open Publication No. 9-262470 has disclosed a producing method of a catalyst utilizing a reversed micelle method. In this method, there is firstly prepared reverse micellar solution including catalytic active components (noble metal element, alkaline metal element, rare earth element) (emulsion solution preparing step), the emulsion solution is then sprayed and fired in air to provide a catalytic powder (spraying and firing step), and catalytic active components are carried on to a substrate (carrying step), followed by firing to obtain a catalyst. However, in this method, catalytic active components are carried on the substrate after spraying the emulsion solution, so that noble metal particles are carried only on the outermost surface of the substrate. This results in a possibility that noble metal particles are sintered due to usage of the catalyst over a long period of time to thereby deteriorate the activity of the catalyst.
Further, Japanese Patent Application Laid-Open Publication No. 2000-42411 has disclosed a method for producing a catalyst by containing an element having an oxygen occluding function into reverse micelles in an emulsion solution preparing step. In this producing method, catalytic active components are carried on a substrate within each reverse micelle contained in the emulsion solution, then reverse micelles are broken, and the obtained precipitate is subjected to steps of filtering, drying, grinding, and firing to thereby establish the catalyst. The catalyst produced by this producing method is capable of not only allowing the substrate to carry thereon the element having the oxygen occluding function but also allowing the outermost surface and surfaces of holes formed in the substrate to carry thereon catalytic active components, thereby allowing an enhanced activity of the catalyst. However, in this producing method, the catalyst is produced by spraying and firing the reverse micellar solution, thereby causing factors of complicated producing steps, a prolonged producing time and a considerably increased producing cost.
As such, Japanese Patent Application Laid-Open Publication Nos. 2001-103606, 2002-24847, and 2002-66335 have disclosed producing methods of catalysts, respectively, each utilizing a reversed micelle method to prepare reverse micellar solution followed by spraying and firing. In each of the catalyst producing methods, reverse micellar solution is prepared, catalytic active components are carried on a substrate, and the emulsion solution is sprayed and fired. Thus, the producing step can be simplified and the producing time is shortened, thereby allowing a decreased producing cost.